Photosynthesis

Abstract

Photosynthesis is used to synthesize complex organic materials from carbon dioxide, water, and inorganic salts, using light as the source of energy. The purpose of this experiment was to measure the optimum wavelength and light intensity at which photosynthesis occurs. A buffer solution containing DCPIP was mixed and chloroplasts were added. Different wavelengths and light intensities were tested on the solutions, and a control was used to monitor for any change without light exposure. A spectrophotometer was used to measure the absorbance of the liquid after each subsequent light exposure. It was determined that the optimum wavelength was 665nm and the optimum light intensity was 45 uEinsteins/m2/sec. These findings are as one would predict, because the dominant light pigments are chlorophylls a and b which absorb light on the wavelength from 430nm-450nm and 640nm-660nm. As light intensity increases so, does the reaction rate of photosynthesis, heat withstanding, but with the effects of heat also having a factor, after reaching a certain intensity reaction rates drop due to inhabitance from the heat-sensitive enzyme system.

Introduction

According to Webster’s Dictionary, photosynthesis is “the synthesis of complex organic materials, esp. carbohydrates, from carbon dioxide, water, and inorganic salts, using sunlight as the source of energy and with the aid of chlorophyll and associated pigments” (Webster’s New World, 1999). Photosynthesis is divided up into two groups: 1)light reactions which require the presence of light; and 2)dark reactions which don’t depend on light, but utilize the ATP and NADPH produced by the light reactions to carry out their processes. Both light and dark reactions of photosynthesis take place in the chloroplasts of plant and algae cells (Kent A Vliet, 2007). Chloroplasts are composed of thylakoids, which in stacks become grana, and of stroma. Pigment molecules in the thylakoid membrane absorb light energy using this in the photosynthesis process. The two general pigments are chlorophylls a and b; and accessory pigments. Chlorophylls a and b absorb light from 430nm-450nm and 640nm-660nm while accessory pigments absorb over a broad range. Pigments do not absorb all wavelengths of light and some wavelengths are better than others. DCPIP will be used to measure how fast the reactions are occurring. Over time as it accepts more electrons from the light reactions, DCPIP loses its blue color and becomes clear. The rate of decoloration will serve as a measure of the rate of the photosynthesis. (Kent A Vliet, 2007).

The purpose of this experiment is to investigate the light reactions of photosynthesis, examining two factors: the wavelength of light and intensity of light on the rate of photosynthesis; determining the optimal wavelength and intensity of light for photosynthesis reactions. It is hypothesized that the more intense the light is, the faster the rate of photosynthesis will occur, also that the optimal wavelengths of light will be between 430nm-450nm and 640nm-660nm. It is with this experiment that this hypothesis will be tested to conclude if in fact the optimal wavelength and light intensities are as suspected.

Materials and Methods

To begin the experiment in checking for the effects of light intensities, a blank, experimental and control sample was made. To make the blank: 2ml of PO4 buffer, 4.5ml of distilled water, and 0.2ml of chloroplast suspension was added to a cuvette. The machine was blanked using this throughout the entire experiment. The control and experiment were made as follows: 2ml of PO4 buffer, 2.5ml of DCPIP, 2ml of distilled water, and 0.2ml of chloroplasts. It is important to note that chloroplasts were not added until all lights were off. The light intensities tested were as follows: 3, 7, 15, 35, and 150 uEinsteins/m2/sec. The following procedure is for one of the intensities above, and procedures should be repeated for all indicated intensities. The lab was kept in the dark during all of the following steps, and the cuvettes were kept on ice to keep the chloroplasts alive. After the chloroplasts were added to the cuvette, a time zero reading was taken. The control cuvette was then covered using aluminum foil to protect it from light. The Experimental cuvette was then exposed to a predetermined light intensity for 2 min, after which a new absorbance reading was taken. The control reading was also taken after every time interval as well. The machine was blanked before every reading and the time intervals were as follows: 0, 2, 4, 6, 8, 10, 12, 14, and 16. The absorbance values were recorded onto Table 2.

To begin the experiment in checking for the effects of wavelength, a blank, experimental and control sample was made. To make the blank: 2ml of PO4 buffer, 4.5ml of distilled water, and 0.2ml of chloroplast suspension was added to a cuvette. The machine was blanked using this throughout the entire experiment. The control and experiment were made as follows: 2ml of PO4 buffer, 2.5ml of DCPIP, 2ml of distilled water, and 0.2ml of chloroplasts. It is important to note that chloroplasts were not added until all lights were off. The wavelengths tested were as follows: 450nm, 545nm, 650nm, 750nm. The following procedure is for one of the wavelengths above, and procedures should be repeated for all indicated wavelengths. The lab was kept in the dark during all of the following steps, and the cuvettes were kept on ice to keep the chloroplasts alive. After the chloroplasts were added to the cuvette, a time zero reading was taken. The control cuvette was then covered using aluminum foil to protect it from light. The Experimental cuvette was then exposed to a predetermined wavelength for 2 min, after which a new absorbance reading was taken. The control reading was also taken after every time interval as well. The machine was blanked before every reading and the time intervals were as follows: 0, 2, 4, 6, 8, 10, 12, 14, and 16. The absorbance values were recorded onto Table 1.